Convert Rust reference-style links to Markdown reference-style links

[ok-nick]

Some Rust docs have reference-style links like this:

/// Create a new [`Manifest`].
///
/// [`Manifest`]: crate::Manifest

or if the Manifest is in the scope of the Rust module, you may see:

/// Create a new [`Manifest`]

We should convert these Rust doc references globally to markdown-compatible links.

See for more information:
https://contentauth.github.io/json-manifest-reference/review-questions#reference-style-links

Note this may also be something that can be implemented within the Rust schemars crate itself.